Not applicable.
Not applicable.
(1) Field of the Invention
This invention generally relates to methods of producing fertilizers. More particularly, this invention is directed to a process for removing dissolved metals and metal compounds from an aqueous fertilizer solution, such as ammonium sulfate fertilizer produced as a byproduct of a flue gas desulfurization (FGD) process in which an ammonium sulfate solution is used to absorb sulfuric acid from a flue gas.
(2) Description of the Related Art
Agricultural fertilizers are mined as well as produced in processes in which the fertilizer may be the primary product or a byproduct of the process. Depending on their source, fertilizers may contain contaminants such as metals and particularly heavy metals (e.g., iron, chromium, lead, zinc, copper, nickel, cadmium, etc.) in the form of molecules (metal compounds) or ions from the dissociation of compounds of these metals, e.g., sulfides, hydroxides, etc. High heavy metal contents in fertilizers are undesirable as posing a health risk to humans and the environment. Because fertilizers are marketed with specifications that describe many of the physical characteristics of these products, including metal content, a sufficiently high heavy metal content can negatively affect product value. Some heavy metals, iron particularly, can also undesirably effect the color of the fertilizer product, further reducing product value.
To limit contaminants, mined fertilizers are often selectively mined from certain parts of the deposit seam. For example, potassium chloride has been mined by a process known as solution mining, in which water is injected to dissolve an underground deposit of potassium chloride, after which the solution is pumped to the surface for product recovery. To improve product quality, a mechanical separation technique may be used to remove contaminants. However, various compounds of heavy metals are soluble in aqueous solutions of fertilizers, and therefore cannot be removed by filtering or other standard separation methods.
Contaminant levels in fertilizers produced by chemical processes, such as the production of ammonium sulfate through the reaction of ammonia and sulfuric acid, are typically controlled by the selection or treatment of the reagent feedstocks, though heavy metal contaminants may still be present in these fertilizer products. On the other hand, fertilizers produced as a byproduct of a process may have contaminants from the various streams involved in the process, and from the corrosion of the materials used in the construction of the process facility. An example is the production of ammonium sulfate and ammonium nitrate fertilizers and, with secondary processing, potassium sulfate and potassium nitrate fertilizers, as byproducts of flue gas desulfurization (FGD) systems (scrubbers) that remove acidic gases such as sulfur dioxide (SO2), hydrogen chloride (HCl) and hydrogen fluoride (HF) from flue gases produced by power plants or other combustion sources. Fertilizer byproducts produced by FGD processes are typically in the form of an aqueous slurry containing precipitates of the fertilizer compound, as well as contaminants that were present in the flue gas, particularly as a result of the various metal oxides that are present in the fly ash and then transferred at least in part to the fertilizer stream. The control of contaminants in fertilizers produced by FGD has been typically through the removal of fly ash from the flue gas stream, and by filtering or other mechanical separation of the fertilizer solution. However, heavy metal compounds typically present in the fertilizer byproduct of FGD are often dissolved, and therefore cannot be removed by filtering or mechanical separation. While contaminated waste water streams from mining sites have used lime (CaO) to adjust the pH of the waste water, causing the precipitation of metals from the stream, the use of lime is not appropriate for FGD processes because lime reacts to form calcium salts that are not desirable as fertilizers. Also, as a solid, lime is not convenient as an addition to treat contaminated fertilizers.
In view of the above, it would be desirable if a method were available that could reduce the metal content of fertilizer products, and particularly fertilizer byproducts of FGD processes, to a level that will essentially eliminate the risk to humans and the environment, as well as enhance the value of the fertilizer.
The present invention provides a process and facility for producing fertilizers with reduced levels of contaminants as a result of the fertilizer stream being treated to precipitate undesirable metals. The invention is particularly directed to flue gas scrubbing systems that produce a useful fertilizer byproduct, in which the fertilizer byproduct is treated to remove heavy metals originally present in the scrubbed flue gas.
The process of this invention generally entails contacting a flue gas with a scrubbing solution to remove acidic gases from the flue gas and produce a byproduct containing a fertilizer compound (dissolved or precipitates) and dissolved metals and/or metal compounds. If fertilizer compound precipitates are present, water is added to the byproduct to dissolve the precipitates, such that the byproduct is a fertilizer solution containing dissolved fertilizer and dissolved metals and/or metal compounds. The pH of the fertilizer solution is then adjusted with a gaseous and/or liquid alkali to form precipitates of one or more metal species, i.e., metal precipitates or compounds of the metals. The pH of the solution is adjusted with ammonia or another alkali that will not contaminate the fertilizer byproduct and is generally compatible with the flue gas scrubbing process. Multiple pH adjustments are preferred to selectively precipitate individual metal species from the fertilizer solution, generally in a pH range of about 7 to 12. Depending on the chemistry of the dissolved fertilizer compound, the treatment of the fertilizer solution with an acid such as sulfuric acid or hydrochloric acid is also within the scope of the invention for the purpose for decreasing the pH of the fertilizer solution to a level necessary to precipitate compounds of particular metal species. The precipitates can then be removed from the fertilizer solution by known filtering or mechanical separation methods.
A flue gas scrubbing facility configured to perform the above process generally comprises a gas-liquid contactor or other suitable means for contacting a flue gas with the scrubbing solution, such that acidic gases are removed from the flue gas. Suitable vessels or other means are also provided with which the fertilizer solution is formed, the pH of the fertilizer solution is adjusted with the gaseous and/or liquid alkali to form the precipitates of the metal specie(s), and the precipitates are then removed from the fertilizer solution.
In view of the above, the present invention provides a process and facility that are capable of significantly reducing the metal content of fertilizers, and particularly fertilizer byproducts produced by flue gas scrubbing operations, such that such operations are able to produce a useful byproduct that is safer for the environment. The invention achieves this result by dissolving the fertilizer compound of the byproduct, such that a solution is formed containing dissolved fertilizer compound as well as dissolved compounds of the metals that are desired to be removed. Accordingly, an important aspect of the invention is the solubility of the particular fertilizer compound (e.g., ammonium sulfate, ammonium nitrate, potassium sulfate, potassium nitrate or potassium chloride) in the presence of the agent (e.g., dissolved ammonia) used to precipitate the metals. In general, higher ammonia concentrations reduce the solubility of these fertilizers, such that judicious use of the precipitation agent is required to avoid significant precipitation of fertilizer that would then be lost in the process of separating the metal precipitates from the fertilizer solution. Finally, the precipitation agent should be recoverable from the fertilizer solution, an acceptable constituent of the fertilizer byproduct, or compatible with the flue gas scrubbing solution so as to allow a portion of the fertilizer solution to be returned with the added precipitation agent to the scrubbing process. In view of these considerations, ammonia is a particularly preferred agent for precipitating metals for fertilizer solutions. A suitable method for recovering ammonia from a fertilizer solution would entail stripping the ammonia and returning the ammonia to the pH adjustment step. Alternatively, some or all of the stripped ammonia may be returned to the scrubbing process, as can be some or all of the ammonia-rich fertilizer solution if the scrubbing solution employs ammonia.
Other objects and advantages of this invention will be better appreciated from the following detailed description.